Successful Water Based Drilling Fluid Design for Optimizing Hole Cleaning and Hole Stability

Abstract

fax 01-972-952-9435. AbstractWater based drilling fluids have been used in many North Sea drilling operations during the last decades. Traditionally, a high focus has been directed towards creating inhibitive drilling fluids with high low shear rate viscosity for optimising cuttings transport and intact cuttings on the shaker. It has, however, been recognized that there is no direct connection between cuttings quality and hole stability. In the current field it was also found that a low gel/viscosity drilling fluid is effective in transporting cuttings out of the wellbore. Following this, the content of high molecular weight polymers have been reduced and exchanged with low molecular weight polymers to enhance hole cleaning throughout the last years. Fewer cuttings are entrapped in beds down hole and are transported up to the surface. The current paper demonstrates how this low viscous/low gel strength drilling fluid contributed to improved drilling performance, where an improvement in tripping time, casing running speeds and total drilling time was observed.In addition to changes in the viscosity a wide range in NaCl and KCl salt concentrations and mixtures have been tried and field tested. The project is currently using only a moderate concentration of KCl, which has improved hole stability. The frequency of unstable hole occurrences in a particular troublesome formation are today greatly reduced by using lower KCl salt concentration and improved well design.The current paper explains in detail how the drilling fluid design optimizes the chemical performance and the hole cleaning performance during the drilling operation. The drilling performance for this particular field improved significantly during the last three years where the total drilling time was reduced with roughly 50% compared to earlier practice. The reasons for the improvements are mainly the changes in well design combined with the use of an effective WBM system.

Hole Cleaning Performance of Light-Weight Drilling Fluids During Horizontal Underbalanced Drilling

“…Therefore, fluid-flow velocity is the only variable that has direct impact on the shear stress acting on the cuttings bed surface. Thus, the major variable influencing the hole cleaning performance is actually the shear stress, which agrees with the previously conducted studies on this area (5)(6)(7) .…”

“…The major factors affecting the carrying capacity of drilling fluids may be listed as fluid annular velocity, hole inclination, drilling fluid properties, penetration rate, pipe/hole eccentricity, hole geometry, cuttings properties and drillpipe rotation speed (4) . In fact, fluid-flow velocity is the dominant drilling variable on hole cleaning because of its direct relation to shear stress acting on the cuttings bed (5) . It has been stated that in order to remove cuttings from a horizontal or deviated wellbore, a sufficient shear stress should be applied to the cuttings bed surface in order to lift the particles and erode the developed bed.…”

“…dependent on not only the fluid properties, but also the cuttings' properties, such as shape, compaction properties, etc. (5)(6)(7) Additionally, it is reported that because the interaction between the drilling fluids, cuttings and gel formation within the developed cuttings bed occurs, which significantly increases the required shear force needed to erode the bed, the cuttings particles are lifted up from the bed (6,7) . Studies on cuttings transport have been in progress over the past 50 years (2) .…”

Cationic surfactant (CTAB) a multipurpose additive in polymer-based drilling fluids

“…Polymer-based drilling fluids are used to drill pay zones, horizontal and multilateral wells due to the clean and minimum formation damage property. Polymer mud formulated with KCl, xanthan gum, starch and PAC-LV shows an improved hole cleaning and hole stability property (Simonides et al 2002;Kjosnes et al 2003;Patel et al 2007). Xanthan gum has got unique rheological properties, such as high lowshear viscosity and strong shear thinning character in the solution.…”

“…This can have negative impact on rheological properties when exposed to high-pressure and high-temperature (HPHT) condition and contaminated with other minerals, which are common in deep drilling. High-pressure and high-temperature (HPHT) wells have bottom hole temperatures of 300°F (150°C) and bottom hole pressures of 10,000 psi (69 MPa) or higher (Saito et al 1998, Saito andSakuma 2000;Kjosnes et al 2003). Present work is aimed to study the effect of CTAB, a cationic surfactant, on control fluid loss, other biocide use elimination, improvement in rheological property and thermal stability of a polymer drilling fluid.…”